Why do we wear costumes and masks on Purim? Well, it could be to remind us that Queen Esther hid her Jewish identity from King Ahasuerus. Because of that, she was able to save the Jewish people. It could be a way for us to turn the world upside down for a little while, in the same way that the world was turned upside down in Shushan: Haman was hanged on the gallows that had been built for Mordechai; the Jews were not killed, but were able to defend themselves; and a day of mourning was turned into day of joy.
While it is sometimes important, even life-preserving, to "put on a mask," you might want to think about how you live your life day to day.
Do you wear a mask when you go to school? Maybe you put on the mask of the "cool skateboarder dude" or the "giggly popular girl." After a while, it gets hard to keep that mask on. So throw it away and let your beautiful face, the real you, shine through!
Purim takes place on the 14th day of Adar. So we say: Mishenichnas Adar marbim besimcha. "In the month of Adar, we are filled with joy."